3: Bonding Theories

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This chapter discusses the different theories which can be used to describe bonding in molecules. Some of these theories should be familiar from general or organic chemistry and some may be new to you this semester. Not every theory is applicable in every situation so, so we will look at cases where these theories are successful and cases where they fail (can't explain or reproduce experimental observations).

Learning Objectives

Apply Lewis and VSEPR theories to determine molecular geometry and bond angles
Identify polar and non-polar molecules
Model bonding and orbital hybridization with s and p atomic orbitals
Illustrate how atomic orbitals can overlap to form sigma, pi, delta, or non-bonding interactions
Construct a qualitative molecular orbital diagram for a homo or heteronuclear diatomic molecule

The thumbnail image compares the Lewis dot structure with the sigma antibonding molecular orbital in HCl. The molecular orbital was calculated and rendered with WebMO.

3.1: Types of Bonding
In general chemistry we learned that bonds between atoms can classified as ionic bonds (full electron transfer) or covalent bonds (fully shared electrons). However, simple “ionic” and “covalent” bonding are idealized concepts and most bonds exist on a two-dimensional continuum described by the van Arkel-Ketelaar Triangle
3.2: Electronegativity Values
The electronegativity of an atom is a relative value of that atom's ability to attract election density toward itself in a covalent bond. There are a few different 'types' of electronegativity which differ only in their definitions and the system by which they assign values for electronegativity.
3.3: Lewis Electron-Dot Diagrams
The bonding between atoms in a molecule can be topically modeled though Lewis electron dot diagrams. Creating Lewis diagrams is rather simple and requires only a few steps and some accounting of the valence electrons on each atom.
3.4: Valence-Shell Electron-Repulsion Theory
The VSEPR model can predict the structure of nearly any molecule or polyatomic ion in which the central atom is a nonmetal, as well as the structures of many molecules and polyatomic ions with a central metal atom. The premise of the VSEPR theory is that electron pairs located in bonds and lone pairs repel each other and will therefore adopt the geometry that places electron pairs as far apart from each other as possible.
3.5: Molecular Polarity
Dipole moments occur when there is a separation of charge. They can occur between two ions in an ionic bond or between atoms in a covalent bond; dipole moments arise from differences in electronegativity. The larger the difference in electronegativity, the larger the dipole moment. The distance between the charge separation is also a deciding factor into the size of the dipole moment. The dipole moment is a measure of the polarity of the molecule.
3.6: Valence Bond Theory
Valence bond theory uses a process called hybridization, in which atomic orbitals that are similar in energy but not equivalent are combined mathematically to produce sets of equivalent hybrid orbitals that are properly oriented to form bonds.
3.7: Molecular Orbital Theory
Similar to valence bond theory, molecular orbital theory involves the mixing of atomic orbitals, but rather than mixing multiple atomic orbitals on the central atom to form hybrid orbitals atomic orbitals from multiple atoms are mixed together to form molecular orbitals.